As packet technologies grow mature, traditional telecom networks that are based on circuit switching are developing towards broadband telecom networks that are based on packet switching, while the adoption of Session Initiation Protocol (SIP) as the call control signaling of a packet switched core network is one of the trends of technological development. For example, according to the researches on the Next Generation Network (NGN) done by standardization organizations like the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) and European Telecommunications Standards Institute (ETSI), in this new packet telecom network, new packet terminals, that is, SIP terminals, will gradually take the place of traditional telephone terminals.
The Telecommunications and Internet Converged Services and Protocols for Advanced Networking (TISPAN) under ETSI has opened a number of Work Items (WIs) to study the services provided to SIP terminals in the NGN system. For example, WI 03023 (DTS/TISPAN-03023-NGN-R1 NGN CONF) carries out researches on the conference (CONF) service, which enables a user to initiate, participate in and control a conversation involving multiple parties.
FIG. 1 shows a signaling procedure for implementing the conference service, which is specified in Draft ETSI TS <3023> V<0.0.19> (2005-12) of WI 03023. The signaling procedure includes the following steps:
S102: User A initiates a dialog to call a Public Switched Telephone Network/Integrated Services Digital Network (PSTN/ISDN) user.
S104-S128: A dialog is set up successfully between user A and the PSTN/ISDN user, and they start a conversation.
The interaction in Step S108 is to reserve resources and the interaction in step S116 is to set up the dialog.
S130: User A requests a conference by initiating a dialog to an Application Server (AS), a service control network element that processes the conference service.
S132-S148: The conference requested by user A is created successfully.
The interaction in step S136 is to create the conference.
S150: User A sends a SIP REFER message to the conference AS, inviting the PSTN/ISDN user conversing with user A to join the conference.
S152-S160: The SIP REFER arrives at the conference AS and the conference AS sends a SIP NOTIFY to notify user A of the reception of the SIP REFER.
The interaction in step S154a is to reserve resources for the new conference participant.
S162: The conference AS sends a SIP INVITE to the PSTN/ISDN user to initiate a dialog according to information carried in the received SIP REFER.
S164-S182: Because the PSTN/ISDN user is a traditional circuit switched domain user, the interworking between the circuit switched domain and the packet switched domain is processed by a Media Gateway Control Function (MGCF). Therefore, the SIP INVITE is sent to the MGCF, and the MGCF acts on behalf of the PSTN/ISDN user to create a dialog with the conference so that the PSTN/ISDN user can be added to the conference. Afterwards, the conference AS sends a SIP NOTIFY to notify user A that the action requested by the SIP REFER is successful.
The interaction in step S166 is to hand over an existing transmitting channel to a new Real-time Transport Protocol (RTP) dialog; the interaction in step S170a is to add the new conference participant to the conference; the interaction in step S182 is to hand over the existing transmitting channel to the new RTP dialog.
S184-S194: The MGCF releases the dialog denoted by the Dialog identifier set in the Replaces information in the SIP INVITE. This dialog is the one already set up between user A and the PSTN/ISDN user.
A detailed description of the above procedure is available in the related draft of WI 03023.
According to the foregoing procedure, a user needs to first create a conference and then invite other participants to the conference after the conference is successfully created. This conference implementation procedure is subject to the following disadvantages:
1. The steps are complex and the conference creation is inefficient.
Complex steps are required for user A to initiate a conference and add participants to the conference. User A needs to first request a conference via a message like SIP INVITE and then, after the conference is created successfully, invite a participant to join the conference via another message like SIP REFER.
2. Relations between conference and other services are not considered.
An already established conversation may trigger the application of many other services. The service control elements that process these services are already on the signaling path of the conversation. When the procedure shown in FIG. 1 is adopted to create a conference, the conference AS establishes two new dialogs respectively with user A and the MGCF and releases the conversation already established between user A and the MGCF. In this way, services triggered on the already established conversation are also released.
For example, because the terminal used by user A has no international call authority, user A uses a prepaid card to establish an international conversation and a service control element that processes prepaid services exist on the signaling path of the conversation. When user A initiates a conference call, because the already established conversation is released, the prepaid service control element does not exist on the signaling path of user A so that the conference is unsuccessfully created (due to the lack of international call authority), or user A may initiate international calls without a prepaid card.
For another example, in an already established conversation, or when the call of a conversation is initiated, user A activates a service, such as the advice of charge. After a conference is created, user A will not receive advice of charge from the network unless user A re-activates the advice of charge service when user A initiates the conference call or after the conference is created successfully.
User A, however, does not expect changes to the prior service experience but hopes to hold services already in application.